Recent Publications

AI-based plasma equilibrium parameter inference in KSTAR for DEMO-relevant reactors using only ex-vessel magnetic diagnostics

Jeongyeon Nam, Jeongwon Lee, Jun Gyo Gyo Bak, Jayhyun Kim, Hyunsun Han, Yong Un Nam

This study proposes a deep neural network for high-fidelity equilibrium parameter inference, serving as a surrogate of the magnetic EFIT reconstruction and relying exclusively on ex-vessel magnetic signals. The objective is to circumvent the long-term integrity constraints of in-vessel magnetic diagnostics in the nuclear environment of the Demonstration Fusion Power Reactor (DEMO)-relevant devices. Using KSTAR data (2021-2024), a baseline model trained solely on measured ex-vessel magnetic diagnostics (the Measurement-only model) is compared with an Augmented model, which addresses informational sparsity via virtual-sensor signals computed from the GEQDSK. The Augmented model achieves predictive fidelity comparable to off-line EFIT, with a coefficient of determination (R 2 ) above 0.99 and a range-normalized root-mean-squared error (NRMSE) of 0.332 % averaged over all parameters, against 2.385 % for the Measurement-only model. Because the virtual-sensor inputs derive from the EFIT solutions that define the targets, these figures quantify the surrogate fidelity with respect to the magnetic EFIT and constitute an upper bound on the accuracy attainable with physically installed ex-vessel probes, not that of an independent reconstruction. As the reference floor for the Measurement-only model, the magnetic fitting residuals of the parent EFIT reconstructions against the measured probe signals are quantified, yielding pooled range-normalized residuals of 1.55-3.87 % per campaign with structured, campaign-persistent components that random noise perturbations do not emulate. Robust accuracy is maintained during neutral beam injection (NBI)-induced transients, and Shapley Additive exPlanations (SHAP)-based attribution verifies that the Augmented model captures the underlying magnetohydrodynamic physics, specifically the Shafranov shift. With a median single-forward-pass inference latency of 3.47 ms and a 99th percentile of 3.53 ms, the framework is a viable surrogate for conventional numerical solvers. In conclusion, this study demonstrates that an idealized, EFIT-consistent realization of a densified ex-vessel sensor configuration retains sufficient information for high-precision equilibrium parameter inference, provided that magnetic probes are physically installed at the designated locations. This establishes the information-content prerequisite for ex-vessel-only inference in DEMO. The accuracy attainable from physically installed probes, subject to conductor-model, calibration and structured measurement-model errors, is not established and requires validation against measured ex-vessel signals.

Assessment of drying status in KSTAR PFC cooling channels via transient temperature response analysis

Koung Moon Kim, Soo Hwan Park, Hyunmyung Lee, Jae-in Song, Nam-young Joung, Kwangpyo Kim, Young Min Park

Integration and evolution of architectural components in the ITER plasma control system: deployment and experimental validation at KSTAR

Woong-Ryol Lee, Taehyun Tak, Luca Zabeo, Timo Ravensbergen, Anze Zagar, Piotr Perek, Walter Van Herck, Peter De Vries, Seung-Ju Lee, Lana Abadie, et al.

Plasma GraphRAG: Physics-Grounded Parameter Selection for Gyrokinetic Simulations

Ruichen Zhang, Feda AlMuhisen, Chenguang Wan, Zhisong Qu, Kunpeng Li, Youngwoo Cho, Kyungtak Lim, Virginie Grandgirard, Xavier Garbet

Fabrication Experience and Requirement Comparison of 316L(N)-IG Forging for ITER In-Vessel Components

Heejin Shim, Jaemin Kim, MunSeong Cheon

An FPGA-Based, Full-Digital Phase Comparator of a Multi-chord Interferometer for Real-Time Density Control in KSTAR

June-Woo Juhn, T. G. Lee, S. H. Hahn, K. C. Lee, Y. S. Kim, G. I. Kwon, S. J. Lee, H. Han, Y. H. Lee, Y. U. Nam

Operational stability and beam transport constraints in long-pulse neutral beam injection on KSTAR

J.H. Jeong, S.C. Hong, B.S. Kim, J.Y. Jang, W. Cho, J.G. Kwak, S.J. Wang

Revised error field assessment in KSTAR with profile diagnostics of resonant field penetration

Gwang-Geun Seo, Seongjun Han, Jayhyun Kim, Yeongsun Lee, SeongMoo Yang, June-Woo Juhn, Jong-Kyu Park

Error field (EF) correction against n=1 intrinsic errors has been revised in KSTAR in three respects. First, a local penetration timing criterion is defined from electron cyclotron emission (ECE) and charge exchange spectroscopy (CES) signatures near the reconstructed q = 2 surface. This timing determines the in-vessel control coil current used as the penetration threshold in the compass scan analysis. Second, the compass scans using the MID and OFFMID coils in 1.8T L-mode plasmas indicate an intrinsic EF level larger than the 2015 KSTAR reference under the conditions examined here. This implies that intrinsic error fields can vary after extended periods of device operations and hardware modifications. Third, the inferred EF direction is assessed by comparing the MID and OFFMID results in a GPEC overlap field basis, where the two configurations give nearly the same direction despite their different poloidal spectra. A rotating resonant magnetic perturbation (RMP) experiment provides an additional dynamic check of this direction, with penetration observed near the phase predicted from the static MID scan. The reversed-B t comparison constrains the possible source symmetry and is more compatible with a PF related contribution than with a TF dominated contribution, although the specific hardware source is not identified. These results provide an updated EF reference for KSTAR 3D-field operation and motivate periodic reassessment of intrinsic EF after extended device operation.

Demonstration of H-mode Error Field Identification in a Single Discharge via Island Healing

E. M. Bursch, A. Xie, J. L. Barr, S. K. Kim, N. C. Logan, S. M. Yang, J. G. Bak, W. Choi, Q. Hu, J. W. Juhn, et al.

Non-disruptive compass scan error field identification via island healing is demonstrated on KSTAR in a single H-mode discharge. This method can allow for up to a 75% reduction in the operational time necessary for a complete compass scan by reducing the required discharges from four to one. Previous experiments on DIII-D, JET, and MAST-U have demonstrated the technique in Ohmic and L-mode but required multiple discharges and strong real-time n=1 magnetics diagnostics close to the plasma that are unlikely to be viable for future devices. The results presented here were made possible by implementing a new set of triggering algorithms relevant to fusion pilot plant operation into the KSTAR plasma control system, along with KSTAR's strong RMPs and long pulse lengths. This represents a significant step towards the disruption-free error field identification method being demonstrated for deployment on future disruption-averse tokamaks, including ITER and fusion pilot plants. Steps needed to close remaining gaps to viability are addressed.

Mechanism of Ionization Avalanche in Tokamak Microwave Gas Breakdown

Jinwoo Gwak, Yeongsun Lee, Jeongwon Lee, Won Ik Jeong, Hyun-Tae Kim, Yong-Seok Hwang, Min-Gu Yoo, Yong-Su Na

Microwave breakdown driven by electron cyclotron (EC) waves provides a non-inductive route to plasma initiation in reactor-scale tokamaks. We introduce a three-dimensional Monte Carlo simulation that, for the first time, self-consistently treats nonlinear wave-particle interactions, atomic collisions, and guiding-center transport. The Monte Carlo simulation unveils the key role of parallel Brownian motion in the ionization avalanche mechanism. The predicted breakdown boundary is validated against KSTAR experiments. This work concludes that microwave gas breakdown will be successful under ITER-relevant conditions at a D$_2$ prefill pressure near 2 mPa with 1 MW of injected EC power.

Revisiting the MRX Electron Current Sheet Width with Semi-Collisional Kinetic Simulations at Hydrogen Mass Ratio

Sung Hyun Son, Adam Stanier, William Daughton, Jongsoo Yoo, Tongnyeol Rhee, Hantao Ji

For over eighteen years, the electron current sheet measured in the Magnetic Reconnection Experiment (MRX) has stood a factor of 2--5 wider than predicted by MRX-like kinetic simulations, and the measured electron force balance has not closed with the classical terms alone. As a consequence, the dominant nonideal terms responsible for breaking the frozen-in condition have remained unexplained. Here, two-dimensional kinetic simulations with binary Coulomb collisions are performed in a cylindrical geometry representative of MRX, at the realistic hydrogen mass ratio $m_i/m_e = 1836$ and at MRX-relevant collisionality. For the first time, these simulations reproduce the measured electron current sheet half-width. The simulated value, $δ_{BT} = 0.744 \pm 0.054$~cm or $6.26 \pm 0.45$ electron skin depths ($d_e$), lies within the experimental range of 5.5--7.5~$d_e$. The electron force balance closes through the classical channels alone: the pressure-tensor divergence supports 76\% of the nonideal electric field and collisional friction the remainder. The historical force-balance deficit reappears only when the simulated layer is sampled at the experimental 3~cm outflow resolution, suggesting that the deficit reflects probe resolution rather than anomalous dissipation. Beyond this reproduction, an analytic model of the layer width is developed that orders the meandering electrons by the coherence of their orbits against collisions. In this model, the Dreicer ratio $E_D/|E_y|$ selects the electrons whose current-carrying motion survives, and the resulting width prediction brackets the measured values across a wide collisionality scan. A discrepancy remains in the width normalized to the local electron gyroradius ($ρ_e$), whose measured value lies a factor of 3--6 above both the model and the simulations.

Precision Machining of M72 Inconel 718 Studs for ITER Vacuum Vessel Gravity Support

G.H. Kim, Y.J. Jung, M.S. Ha, H.S. Kim, Y.G. Jeong, S.G. Han, H.K. Kim

Real-time feedback control of ELM frequency using divertor gas puffing and its effects on tungsten-induced radiation and plasma performance in KSTAR

Minseok Kim, Young-Ho Lee, SangKyeun Kim, Minwoo Kim, Sang-hee Hahn, Hiro J. Farre-Kaga, Ricardo Shousha, Juhyeok Jang, SooHyun Son, Yoon Seong Han, et al.

The edge-localized mode (ELM) frequency ($f_{\mathrm{ELM}}$) was successfully controlled in real time on KSTAR using a proportional-integral (PI) feedback controller, employing a $\mathrm{D}_2$ divertor gas puff as the actuator under tungsten lower-divertor conditions. The controller accurately tracked a two-step target---a 30 Hz increase in $f_{\mathrm{ELM}}$ for 4 s, followed by a 30 Hz decrease for 3 s---yielding mean and median absolute percentage errors of approximately 13% and 12%, respectively. Compared to a reference discharge, the actively controlled shot did not exhibit a significant drop in volume-integrated core radiation, confirming that excessive gas use merely degrades overall plasma performance. However, when contrasted with the exponential increase in core radiation observed in the absence of divertor gas puffing, these results underscore the critical need for real-time optimization. Specifically, divertor gas commands must be actively managed to maintain an $f_{\mathrm{ELM}}$ sufficient for flushing tungsten from the core while maximizing global plasma performance.